High hardness is a material property that improves the performance of wear resistant and ballistic steels greatly. Wear resistant steels (also called as abrasion resistant steels) are used for instance in excavator or loader buckets of earth moving vehicles, in which super high hardness means longer service time of the vehicle component. By high hardness it is meant that the Brinell hardness is at least 450 HBW and especially in the range of 500-650 HBW.
Such hardness in steel product is typically obtained by martensitic microstructure produced by quench hardening steel alloy having high content of carbon (0.30-0.50 wt-%) after austenitization in the furnace. In this process steel plates are first hot-rolled, slowly cooled to room temperature from the hot-rolling heat, re-heated to austenitization temperature, equalized and finally quench hardened (hereinafter RHQ process). Because the relatively high content of carbon, which is required to achieve the desired hardness, the resulting martensite reaction causes significant internal residual stresses to the steel. This is because the higher the carbon content the higher the lattice distortion. This means that this type of steel is very brittle and can even crack during the quench hardening (quench induced cracking). To overcome these drawbacks related to brittleness, nickel is typically alloyed to such quench hardened steels. Also a tempering step after quench hardening is usually required, which however increases the processing efforts and costs. Examples of steels produced in this way are wear resistant steels disclosed in reference CN102199737 or some commercial wear resistant steels.
Reference JP 09-118950 A discloses a method for producing a hot-rolled wear resistant steel having a medium level of carbon (0.20 to 0.40 wt %) by the above-mentioned RHQ process, which includes slab heating, hot-rolling, cooling, re-heating to a temperature in the range Ac3-1250° C. and cooling with the cooling rate not less than 1.5° C./sec so that a martensitic microstructure may be obtained.
However, as commonly understood, the hardness of resulting martensite is solely dictated by carbon content. This means that in order to achieve the desired hardness, one needs certain amount of carbon in the steel, which in turn raises the risks for quench induced cracking and brittleness. Other drawback here is that carbon has the most debilitating effect on weldability of steel, as can be also seen from the following equation of carbon equivalent: CE=C+(Si+Mn)/6+(Cr+Mo+V)/5+(Ni+Cu)/15, in which lower CE means better weldability. For example loader buckets are manufactured by connecting the pieces of quench hardened steel plates by welding, good weldability of the quench hardened steel material is highly appreciated. Therefore there is a need to decrease the carbon content without compromising the hardness.
Also, for instance some of the earth moving vehicles operate at low-temperature use and some of the components of those undergo impact loads. For this reason their toughness, especially low-temperature toughness should be at a satisfying level in certain applications. Despite of relatively expensive nickel alloying, the toughness, especially in low-temperature, should be in certain applications further improved together with reasonable alloying costs to promote the use of super high hardness hot-rolled steels in more demanding applications. In this regard, boron alloying is commonly used practice to accomplish the hardenability of martensitic steels with low alloying costs. The boron alloying however requires use of titanium that can be harmful for low temperature toughness.
Further, as the vehicle components sometimes include shapes made by bending or flanging, the bendability of the steel shall preferably be excellent taking into account high hardness.
Also naturally the processing and alloying costs should be kept as low as possible.
References US 2006/0137780 A1 and US 2006/0162826 A1 disclose an alternative method of manufacturing a hot-rolled steel plate having abrasion resistance that is based on coarse Ti or Zr carbides formed at high temperature. However, the Ti or Zr carbides are detrimental for low temperature toughness. The great hardness of the steel and the presence of the embrittling Ti carbides make it necessary to slow down cooling before the temperature has fallen below Ms temperature so that there is no risk for quench induced cracking.
In addition, reference WO 03/083153 A1 discloses a steel block for the production of injection moldings. To manufacture a mold with this steel, the steel is produced, is cast and hot-rolled or hot-forged in a known manner and cut to obtain blocks. The blocks are austenitized, optionally in the forging or rolling heat, and they are then quenched. The chemical composition of the steel block is optimized for high temperature application rather than low temperature application. Thermomechanically controlled processing (TMCP) in conjunction with direct quenching (DQ) or interrupted direct quenching (IDQ) is an effective process to produce low carbon, low alloyed ultra-high strength structural steels in yield strength range from 900 MPa up to 1100 MPa. The present invention extends the utilization of TMCP-DQ/IDQ process to produce high hardness hot-rolled steel products, such as strip and plate steels (450-600 HB) with high performance.